This invention relates to a net for use in protecting plants from direct sunlight.
Although plants require sunlight in order to grow and thrive, different types of plants have differing requirements as to the amount and type of sunlight. Some plants thrive under direct sunlight, while others, sometimes termed shade plants, require filtered or dispersed light in order not to dry out and wither.
It has been known for many years to use shade-cloth or shading paint in order to protect shade plants from direct sunlight. In particular, white colored nets have been found to have a beneficial effect on the growth and agricultural yield of such plants. The white color reflects and disperses the light, while the net (as opposed to a film or sheet) allows aeration of the plant environment. Colors other than white may also be used in such applications.
The conventional technique for imparting a color to the net is by painting the net fabric using a mineral-based paint. For example, white colored nets are prepared using TiO2. However, a major disadvantage of this method is in the abrasive effect of the mineral on the machinery used to produce the net, such as an extruder (when a polymeric fabric is used) and a knitting machine. Decreasing the amount of mineral used in coloring the fabric decreases the abrasive effect, but also decreases the percentage of shade afforded by the net. It is therefore quite difficult to strike a balance between obtaining a sufficient amount of shade from the net while at the same time preventing damage to the manufacturing equipment.
Foamed synthetic polymer resin films are well known for use in various agricultural and other applications.
JP 06099668, published Nov. 28, 1995, discloses a three-layer film comprising a polyolefin-based synthetic resin impregnated with a foaming agent. The film is cut into strips and stretched to obtain a flat yarn for use in preparing an artificial lawn pile fabric. In order to impart a visual appearance similar to a natural lawn, white luster is prevented by irregular reflection of light on the surface.
JP 54015844 published Feb. 6, 1979, discloses a laminated film which filters and scatters sunlight. The film comprises a foamed synthetic resin film having closed cells of 0.3-5 mm, laminated with a synthetic film coated with aluminum powder. The film may be used to cover a greenhouse.
JP 78041384 published Nov. 2, 1978, discloses a thermally-insulating laminated sheet which comprises a flexible cellular plastic sheet bonded to a reinforcing polyolefin fabric using an adhesive containing aluminum powder. The sheet is used to prevent escape of heat from a greenhouse and the adhesive layer reflects thermal radiation.
JP 5340074 published Dec. 21, 1993, discloses a high strength straw matting for covering the floor. The matting comprises a synthetic resin foaming body having a thickness of 2-5 mm formed as a tape reinforcing material laminated on the back of a synthetic resin made straw matting.
It is an object of the invention to provide a novel colored net capable of providing diffuse light to plants covered thereby and which is minimally abrasive to machinery used in its manufacture.
It is a still further object of the invention to provide a method for protecting plants from overexposure to direct sunlight by using a net which diffuses the sunlight.
In a first aspect, the present invention provides a net for diffusing light comprising threads of flat yarn, characterized in that the flat yarn has a layer comprising a foamed synthetic resin film capable of diffusing light passing therethrough.
The net of the invention provides maximum dispersion of light with minimal damage to the manufacturing machinery, due to the negligible amount of mineral-based paint. For example, a black-colored net having a texture which provides 50% shading allows approximately 50% of the original amount of direct or dispersed light to pass through it. On the other hand, a white net according to one embodiment of the invention which similarly provides 50% shading allows approximately 80% of dispersed light and 50% of direct light to pass through it. By increasing the percentage of dispersed light reaching the plants, an increase in photosynthesis is achieved without the plants being adversely affected by direct sunlight.
The net of the invention may be prepared either by knitting or by weaving. In the case of a knitted net, a typical net will comprise a plurality of threads oriented in a first direction and being essentially equal spaced from one another, and having wefts oriented in a second direction which is perpendicular to the first direction, the threads and wefts being interlocked by stitches. The wefts will generally comprise the flat yarn of the invention. The threads may be of similar composition to the wefts, or may be any mono-oriented synthetic resin. Such a net may preferably be prepared by a Raschel knitting method which is known xe2x80x9cper sexe2x80x9d, in which the filaments are attached to knitting elements which comprise two needles and knock-over comb bars which are positioned opposite to one another, and comprising ground guide bars, pattern guide bars and stitch comb bars. An example of such a knitted net is described in European Patent No. 0 723 606, assigned to the Applicant, whose contents are incorporated herein by reference. In another embodiment, threads of flat yarn according to the invention are inserted into the knitted fabric.
In the case of a woven net, either the warp or the weft or both may have the characteristics of the flat yarn described above. In the present application, the term xe2x80x9cweftxe2x80x9d when used with respect to a woven net may refer either to the warp or to the weft, or to both.
The net is knitted or woven to a desired texture, depending on the percentage of shade required. The texture may be determined by the distance between the threads.
As stated above, the wefts consist of flat yarn. Flat yarn is characterized by a large ratio between the width and thickness of the yarn. Typical values for this ratio are 10:1 to 200:1. This provides a large surface area for light diffusion. In the case of a knitted net, the yarn must be of sufficient strength to be knitted. Preferred strength requirements are no less than 20 mN/Den tenacity and an elongation-at-break of between 10% and 60%. The yarn will preferably have a thickness in the range of 20-120xcexc and a width in the range of 0.5-5.0 mm.
The flat yarn may be mono-layered or multi-layered, with at least one foamed layer contributing the optical properties of the yarn (i.e. diffusion of light) and at least one layer contributing the mechanical properties (i.e. structural strength). In the case of a multi-layered yarn, the layers may be extruded together, or manufactured separately and subsequently bound together to form a laminate. The yarn may be manufactured by any of the known conventional methods. In the case of a mono-layered yarn, one layer will combine the optical and mechanical properties.
The synthetic resin film may be prepared from polymers such as polyolefins, polyesters, polyamides, polyimides and other polymeric film making substances, as well as mixtures of different polymers. Examples of polymers which may be used in the net of the invention include, but are not limited to, polypropylene and polyethylene.
As stated above, the synthetic resin contained in the flat yarn is foamed. Foams may be generated in synthetic plastic by a number of methods. These include, but are not limited to, the incorporation of a gas-generating agent in the resin film, expansion of small beads of thermoplastic resin containing a blowing agent, dispersion of gas by mechanical means, and incorporating an inorganic granule additive in the resin film followed by stretching of the film. A preferred size of the granules is 0.5-2.0xcexc. The stretching may be mono-oriented or bi-oriented. In a preferred embodiment, the aforementioned stretching method is used to prepare the foamed synthetic resin sheet for the net of the invention. The sheet may be manufactured, cut into strips for manufacture of the flat yarn and then stretched, or first stretched and then cut into strips. The cavities formed in the resin film contribute to the diffusion of the light. The resin film may be of any color other than black. In a preferred embodiment, the resin film imparts a white appearance to the yarn and net.
In order to impart mechanical strength to the yarn for the knitting or weaving process, at least one layer must be structurally strong, preferably by mono-oriented stretching. In the case of a multilayer yarn, the second layer may be made from the same materials listed above for the foamed synthetic resin. However, this second layer is not foamed, as the foaming generally weakens the mechanical strength of the resin. Most preferably, the second layer is a high-density polyolefin film such as mono-oriented, high-density polyethylene (HDPE). A preferred density of this layer is e.g.  greater than 0.9 gr/cm3 for polypropylene and  greater than 0.94 gr/cm3 for polyethylene. In the case of a mono-layered yarn, a balance must be achieved between foaming and stretching, as is known to the skilled man of the art.
At times it may be advisable to supplement the polymers with various chemical additives such as UV stabilizers and processing agents.
In a second aspect, the invention provides a method for protecting plants from overexposure to sunlight comprising covering the plants with a net according to the invention. The manner of covering the plants is well known to the skilled man of the art and depends on the type of plant, season, geographical area, texture of the net, etc. Use of the net of the invention instead of a black net in covering the plants can increase the amount of diffuse light reaching the plants, resulting in more intensive photosynthesis. The net of the invention may be used, for example, in a permanent rather than a seasonal spread, as opposed to a conventional black net.